Technical Field
The present invention relates to a resonance circuit used for a measurement device, which is configured to measure a physical quantity of a subject to be measured by vibrating an object, and the measurement device.
Related Art
As measurement devices, which are configured to measure a physical quantity of a subject to be measured by vibrating an object, such as the subject to be measured or a measurement piece, a Coriolis mass flowmeter is known. The Coriolis mass flowmeter is a measuring instrument using a Coriolis force acting when vertically vibrating a measurement tube, through which a fluid to be measured flows, with both ends thereof supported, and is configured to measure a mass flow rate of the fluid to be measured based on a phase difference between upstream and downstream sides of the measurement tube vibrating at a natural frequency thereof. The Coriolis mass flowmeter can also measure a density of the fluid to be measured flowing through the measurement tube by measuring a vibration frequency of the measurement tube.
As described in Patent Document 1, an amplitude of resonance is advantageously controlled depending on a diameter. However, if a circuit as described in Patent Document 1 is constructed with an analog circuit, the number of components, such as operational amplifiers, analog switches, calculating resistors and capacitors, constituting a filter, is increased, thereby causing a problem in that an area of a circuit board has to be increased and also the costs is increased. Contrarily, if distal signal processing is employed, an IC in which gate arrays, DSPs (Digital Signal Processors) and the like are integrated can be used, thereby reducing an area of the circuit board and lowering the costs.
In the Coriolis mass flowmeter requiring a high precision measurement, if a resonance circuit is constructed with a digital circuit, an AD convertor IC of a high precision ΔΣ type may be used when a digital signal to be inputted to the resonance circuit is generated.
In general, because the ΔΣ-type AD convertor IC is equipped therein with a ΔΣ modulator and a digital low-pass filter (LPF), a delay is occurred when the LPF performs processing. Therefore, there is a problem in that a phase shift is caused when performing excitation by resonance, so that precision of vibration control is deteriorated and thus precision of measurement is also deteriorated.
In order to solve such a problem, Patent Document 2 discloses a Coriolis mass flowmeter having a high precision and stabilized resonance circuit, in which a delay is reduced while performing digital signal processing.
FIG. 9 is a block diagram showing a main configuration of the Coriolis flow mass meter described in Patent Document 2. A detector 510 has first and second sensor 512 and 514 for measuring displacements of upstream and downstream sides of a measurement tube (not shown), such as a U-shaped tube or a straight tube, through which a fluid to be measured flows, and a exciter 516 constituted of a driving coil and the like.
A pair of analog displacement signals outputted from the first sensor 512 and the second sensor 514 are respectively ΔΣ-modulated by a first ΔΣ modulator 520 and a second ΔΣ modulator 522 and thus become 1-bit pulse density signals.
The pulse density signals are respectively converted to multi-bit signals (ordinary digital data) by a first LPF 524 and a second LPF 526. Two multi-bit signals are sent to the signal computing module 528 so that a mass flow rate and a density are calculated therefrom by a known technique.
The first ΔΣ modulator 520 and first LPF 524 and the second ΔΣ modulator 522 and second LPF 526 constitute, respectively, ΔΣ-type AD convertors, and delays are occurred in the first LPF 524 and the second LPF 526.
An excitation circuit 530 is a circuit for driving the exciter 516 to excite the measurement tube. The excitation circuit 530 is operated by a pulse density signal outputted by the first ΔΣ modulator 520 and a multi-bit signal outputted by the first LPF 524.
The excitation circuit 530 includes a resonance circuit 532 for generating an excitation signal based on output signals of the sensors and a drive output module 534 for amplifying and returning the excitation signal to the exciter 516.
In the resonance circuit 532, a pulse height (level) of the pulse density signal outputted by the first ΔΣ modulator 520 is amplified by a multiplier 540. An amplification factor in the multiplier 540 is determined depending on an amplitude of the measurement tube. Namely, the smaller the amplitude of vibration than a target value, the higher the amplification factor is set. Also, if the amplitude comes close to the target value, the amplification is set to be close to 0.
Specifically, a first HPF 542 cuts a DC signal (offset signal) from the multi-bit signal outputted by the first LPF 124 to extract a vibration signal, which is an AC signal corresponding to vibration of the measurement tube. Then, an amplification factor controller 544 performs a proportional control based on the vibration signal and sets the amplification factor of the multiplier 540 so that the amplitude of the measurement tube is stabilized to a target value.
FIG. 10 is a block diagram showing a configuration of the amplification factor controller 544. The amplification factor controller 544 is configured so that an absolute valve of the AC signal outputted from the first HPF 542 and corresponding to vibration of the measurement tube is taken by an absolute circuit 560 performing rectification. Also, the fourth LPF 562 cuts a high frequency from the value, thereby smoothing the value. Therefore, an amplitude signal which is a DC signal corresponding to an amplitude of the measurement tube is obtained.
Also, a subtractor 564 calculates a difference value between the amplitude signal and a target amplitude value. The difference value is amplified by a variable amplifier 566 and then is set as an amplification factor for the multiplier 540.
Namely, a proportional control is performed in such a manner that if the amplitude signal is smaller than the target value, an output of the amplification factor controller 544 is increased and a gain of the resonance circuit 533 is increased, and on the other hand, if the amplitude signal comes close to the target value, the output of the amplification factor controller 544 comes close to 0 and the gain of the resonant 532 is decreased.
Meanwhile, a register 568 memorizes therein a cutoff frequency to be used in the fourth LPF 562, a target value to be used in the subtractor 564, an amplification factor (proportional gain) to be used in the variable amplifier 566. These values can be changed depending on a diameter of the measurement tube, thereby allowing a more stabilized control.
Returning to the description of FIG. 9, a multi-bit pulse density signal having a pulse height adjusted by the multiplier 540 is again ΔΣ-modulated by a third ΔΣ modulator 546 and thus becomes a 1-bit pulse density signal. If the pulse height is amplified by 1.2 times in the multiplier 540, the pulse density in the third ΔΣ modulator 546 becomes 1.2 times of itself, and also if the pulse height is amplified by 0.8 times in the multiplier 540, the pulse density in the third ΔΣ modulator 546 becomes 0.8 times of itself
The pulse density signal outputted by the third ΔΣ modulator 546 is inputted to a DAC 548 and thus is converted to an analog signal. Then, the analog signal is inputted as an excitation signal to the drive output module 534 after a high frequency component (quantization noise) is removed therefrom by a third LPF 550 and also a DC signal is cut therefrom by a second HPF 552. The drive output module 534 amplifies the excitation signal to drive the exciter 516. Due to such a series of operations, excitation by resonance is performed.
As described above, in the Coriolis mass flowmeter described in Patent Document 2, the amplification factor of the pulse density signal outputted by the first ΔΣ modulator 520, in which a very small delay is occurred, is determined based on the output of the first LPF 524 in which a delay is occurred. Namely, the output of the first ΔΣ modulator 520 is used for a signal which is a reference for the excitation signal and in which a phase shift is not acceptable, and the output of the first LPF 524 is used for setting an amplification factor which is less influenced by a delay. Therefore, in the case of digital control, also, a high precision excitation by resonance having a reduced delay can be performed, thereby obtaining a stabilized amplitude.
Patent Document 1: Japanese Patent Application Publication No. 2003-302272
Patent Document 2: Japanese Patent Application Publication No. 2012-88235
As described above, in the based on-art Coriolis mass flowmeter, the amplification factor controller 544 for performing amplitude control sets an amplification factor for a signal to be returned to the exciter 516 using the proportional control.
However, setting of the amplification factor using the proportional control is likely to leave a steady-state deviation between an actual amplitude and a target value. On the other hand, if the amplification factor is increased to reduce the steady-state deviation, the control system is likely to be made unstable.